There are numerous catalyst systems that are capable of bringing about polymerization of ethylene and/or 1-alkenes. Thus, for instance, so-called Phillips and Ziegler-Natta systems can be distinguished. Of these, a number relate to polymerization in the gas phase. Others aim at polymerization in the presence of a liquid dispersant. The latter can be subdivided into the so-called suspension system, with polymerization taking place at temperatures that are below the temperature at which polyethylene dissolves, and the so-called solution system, with a polymerization temperature that is higher than the temperature at which the formed polyethylene dissolves. Solution polymerization requires special catalyst systems as the catalyst activity and the molecular weight of the produced polymer generally decrease with increasing polymerization temperature. It was not until the end of the sixties that a catalyst was developed the activity of which was such that solution polymerization of ethylene could be effected without having to remove catalyst residues from the product (GB-A No. 1,235,062).
In general, polymerization takes place at temperatures that are only little above the temperature at which polyethylene dissolves, because the activity of catalysts customarily applied so far decreases at high polymerization temperatures. At unchanged residence time, this means that the polymer yield decreases, as a result of which the amounts of catalyst residues in the polymer increase and it soon becomes necessary to wash out the polymer. A problem in this exothermic polymerization reaction is the dissipation of the heat of polymerization. Cooling through the wall or by cooling devices in the reactor may easily lead to polymer deposition on the cooling surfaces, especially at cooling agent temperatures below 150.degree. C. For this reason, strong cooling of the reactor feed was preferred. This, however, costs much energy and will become more expensive as fuel prices rise.
Polymerization at high temperatures would entail energy advantages also in another respect: not only can the strong cooling of the reactor feed be reduced or even be done without, in addition there no longer is any need to heat the product during processing of the polymer in order to evaporate the solvent. The reason for this is that the heat of evaporation decreases or even becomes zero as the solution temperature is higher and approaches or even reaches or exceeds the critical temperature of the solvent, and as a result the enthalpy of evaporation becomes minimal.
For the above reasons there is much demand for high-temperature catalysts. These catalysts should be so active as to retain sufficient activity also at very high polymerization temperatures (in excess of 180.degree. C.). This requirement is rendered even more severe by modern legislation, which imposes clear-cut limits as regards the concentration of transition metals in products. Moreover, the polymer produced is to meet the customary requirements as regards processability and applicability, which implies the molecular weight must be sufficiently high, or the melt index sufficiently low.
European patent applications EP-A No. 57050 and EP-A No. 131 420 describe catalyst systems that are active at very high polymerization temperatures.
The catalyst system of EP-A 57050 comprises the combination of two components, the first of which is prepared by heeating an organoaluminium compound, titanium tetrahalide and, optionally, vanadium oxytrihalide for at least 5 seconds to at least 150.degree. C., and the second of which is an organoaluminium compound. In EP-A No. 131 420 the first component is the same as in EP-A No. 57050, while the second is an alkyl siloxalane. The various components of these catalyst systems are mixed such that in the first component the atomic ratio of aluminium to titanium plus vanadium is between 0.2 and 2.0, and preferably more titanium than vanadium is present. The optimum titanium:vanadium ratio is 85:15. The atomic ratio of the aluminium from the second component to titanium plus vanadium is at most 3.
A disadvantage of these catalysts is that heating of the first component or a portion thereof prior to combination with the second component requires extra energy and is laborious. For industrial-scale polymerization, streamlining of the process is of prime importance. Intermediate heating of a portion of the catalyst system would interfere with this objective. In addition, a precipitate is formed on such heating, which may result in problems with the catalyst feed to the reactor.